Afterimage effects in which a visual response is produced after a visual stimulus has been removed are known but not well understood.
U.S. Pat. No. 4,953,968 issued to Sherwin et al on Sep. 4, 1990 teaches a method and apparatus for testing potentials evoked by visual stimuli; U.S. Pat. No. 5,061,059 issued to Horn on Oct. 29, 1991 teaches a method and apparatus for visual field testing and U.S. Pat. No. 3,772,511 issued to Marban on Nov. 13, 1973 teaches apparatus for simulating a flickering light. However none of the above mentioned patents are indicative of the present invention.